Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 34.6 Mammals Are Amniotes That Have Hair and Produce Milk
Mammals are amniotes characterized by hair and milk production. These features, along with advanced thermoregulation, brain development, and parental care, have enabled mammals to thrive across every major habitat on Earth.
Derived Characters of Mammals
- Mammary glands: produce nutrient-rich milk for offspring
- Hair and fat: insulate against heat and water loss
- Kidneys: conserve water while removing nitrogenous waste
- Endothermy: supported by efficient respiratory and circulatory systems (4-chambered heart)
- Large brain and advanced learning abilities
- Differentiated teeth: incisors, canines, premolars, molars—adapted for varied diets
- Extended parental care: supports learning and survival skills
Example: The kangaroo rat survives in deserts via dry waste, nocturnal habits, and deriving water from food.
Evolutionary Origins
- Mammals evolved from synapsids with a single temporal fenestra (hole behind the eye)
- Early synapsids laid eggs, had sprawling limbs, and no hair
- Over time:
- Jaw bones were repurposed into middle ear bones
- Mammalian traits evolved gradually over 100+ million years
- First true mammals appeared in the Jurassic and stayed small during dinosaur dominance
- After the Cretaceous mass extinction, mammals diversified rapidly
Three Major Lineages of Living Mammals
1. Monotremes
- Lay eggs; found only in Australia and New Guinea
- Include platypus and echidnas
- Have hair and produce milk, but lack nipples—young lick milk from skin
2. Marsupials
- Give birth to underdeveloped young that complete development in a marsupium (pouch)
- Include kangaroos, koalas, opossums
- Found mainly in Australia and South America
- Share nipples and placentas with eutherians (less complex placenta)
- Show convergent evolution with eutherians in similar roles
3. Eutherians (Placental Mammals)
- Embryo develops fully in the uterus
- Have complex placentas and longer pregnancies
- Diverse: includes rodents, primates, carnivores, ungulates
- Diversified quickly in the Cenozoic era after dinosaur extinction
Primates: Our Mammalian Order
- Includes lemurs, monkeys, apes, and humans
- Key traits:
- Grasping hands/feet, flat nails, fingerprints
- Large brain, short jaw, forward-facing eyes
- Opposable thumb (especially in monkeys and apes)
- Highly social and arboreal (tree-adapted)
- Apes: gibbons, orangutans, gorillas, chimpanzees, humans
- No tails, large brains, complex social behavior
- Humans: show unmatched dexterity, cognition, and social structure
In a Nutshell
Mammals, defined by milk and hair, evolved from synapsid ancestors and diversified into monotremes, marsupials, and placental mammals. These adaptations allowed mammals to radiate across Earth’s ecosystems. Primates, one mammalian group, evolved features for tree-dwelling life, and from them emerged the human lineage—with unprecedented brain power and social complexity.